FIG. 1 is a perspective drawing of a substrate assembly (1) with six chips (10,11,12,13,14,15) and a heatsink hat assembly (2) with space for six pistons, five of which spaces are populated (3, 4,5,6,7) and one of which is missing a piston (8). Each piston is spring loaded.
During packaging, a substrate and a heatsink hat assembly are bonded together. More particularly, each piston is seated on a chip in order to provide heat dissipation through the hat assembly. It is critical that each chip in the array have a corresponding piston on the heatsink. If a piston is missing (as for example, in FIG. 1, 8), the matching chip (15) will have no way of dissipating heat and would fail (thermal breakdown). Conversely, it is critical that each piston have a corresponding chip. A piston lodged against an empty chip socket could cause a short between contacts on the chip socket. Thus, any mismatch results in a chip defect and (potentially) either a required scrapping of the entire assembly or expensive rework.
In prior art, the piston and chip were matched visually by the operator. If a chip was not present on the substrate assembly, the operator would manually remove the corresponding piston and spring. The visual matching operation was prone to human error and often resulted in the destruction of extremely expensive components.